The present invention relates, in general, to a metering device for an injection molding unit.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Injection molding of plastic materials requires injection of a precise amount of plastic material (shot amount) per cycle (shot) into the cavity of a molding tool, and constant reproducibility when injection of several shots is intended. This is especially true when producing data cards such as bank cards or so-called “smart cards”, or also data carrier such as CDs, DVDs etc. because of the need to keep very narrow tolerances. This problem is of even greater concern in conjunction with injection compression molding with multiple cavity molds which involves injection of plastic material in a cavity that is enlarged to suit a compression stroke and subsequent compression of the plastic material. In this case, the precise shot amount must be provided in each cavity prior to the compression process because the cavities close the injection channel during compression so that a subsequent sprue removal is omitted.
Further to be taken into account is the fact that shot amounts vary as a result of thermal unbalances such as temperature control areas, different flow resistances in the hot runner manifold of different cavities as a consequence of manufacturing tolerances or other imprecision in the system. As the injection process in connection with injection screws is impacted by mechanical properties of non-return valves or the like, the accuracy of the shot amount is oftentimes inadequate.
It would therefore be desirable and advantageous to provide an improved metering device to obviate prior art shortcomings and to attain a precise injection amount of plastic material for each shot before carrying out the injection process.